Adding to this a bit late, but since it was still listed as "unanswered" I will throw this in:

The Institute for Research on Labor & Employment Library at UC Berkeley seems to have what you're looking for:

The IRLE Library collects information in multiple disciplines and formats about employment and workplace issues. Particular strengths of the collection are labor history, labor and culture, labor and the arts, labor relations, organization studies, international labor issues, trade unionism and collective bargaining.

(Scroll down a bit, and you can see the information organized by state.)